Firing system for jet type perforating gun



J. H. CASTEL FIRING SYSTEM FOR JET TYPE PERFORATING GUN Filed-A ril s. 1951 Jan. 28, 1958 4 Sheets-Sheet l INVENTOR- J. H. CASTEL Jan. 28, 1958 FIRING SYSTEM FOR JET TYPE PERFORATING GUN Filed April 5, 1951 4 Sheets-Sheet 2 Jacques %el I Jan. 28, 1958 J. CASTEL 2,821,136

FIRING SYSTEM FOR JET TYPE PERFORATING GUN Filed April 5, 1951 4 Shets-Sheet s I, g 7 \l l n I I l 25 I I ll I I l l I i l I I -12;

a /z 147 as 3 14% INVENTOR.

' JdCges If. Cas'iel lizforrzgys United es FIRING SYSTEM FOR JET TYPE PERFORATING I GUN Jacques H. Caste], Houston, Tex., assignor, by mesne assignments, to P. G. A. C. Development (10., Houston, Tex., a corporation of Texas Application April 5, 1951, Serial No. 219,480

3 Claims. (Cl. 10221.6)

The present invention relates to perforating guns of the type used for casing perforation work in oil field recovery and well completion operations, and, more particularly, to an improved system for sequentially firing the explosive charges of different gun sections in a socalled jet type perforating gun.

=Conventionally, a jet type perforating gun comprises 'a plurality of tubular gun barrels having explosion ports extending through the wall thereof at spaced points therealong. A shaped jet producing explosive charge is positioned within each of the gun barrels opposite each explosion port so that the explosive forces developed upon detonation of the charge are discharged through the port and perforate the wall of the casing adjacent thereto as well as the formation surrounding the casing, to permit entry of fluid into the borehole. It is desirable to provide a jet type perforating gun comprising a plurality of gun sections, together with suitable means for sequentially firing the same so that the gun sections can be fired at different points along the borehole or successively at the same location in the borehole without removing the entire assembly from the hole for reloading after each salvo is fired. Such an arrangement results in a large saving in the amount of equipment and time required to complete a given perforating job. Moreover, by shooting a given number of shots in groups rather than all at once, it

becomes possible to minimize the fluid surge pressures which are developed in the well fluid. However, a sequential firing system of the character described must be .so arranged that the gun sections can only be fired in a .known predetermined order, since inadvertent firing of the wrong gun section would result in perforation at the wrong location in the borehole. It is also desirable to provide a sequential firing system of the character described which is so arranged that a single conductor cable :may be used to raise and lower the gun unit within the borehole and control the sequence of firing the shaped sectionalized jet type perforating gun section by section 'in a known predetermined order.

..atent.

It is another object of the invention to provide an improved sequential firing system of the character described which includes no moving parts in the gun unit, is simple in arrangement and reliable in operation.

Another object of the invention is to provide a new and improved firing system of the character described in which one gun section cannot be fired accidentally while trying to fire another section of the gun, and more particularly in which the gun sections can only be fired from the bottom section upward.

It is a further object of the present invention to provide a new and improved system for. sequentially firing a plu- 2,821,136 Patented Jan. 28, 1958 ice rality of jet type perforating gun sections in which a single conductor cable is employed to lower the apparatus as a unit into the borehole and to supply current to the ignition elements of the firing system.

It is a still further object of the present invention to provide a new and improved system for sequentially firing the sections of a two-section jet type perforating gun in which a single conductor cable is employed to lower the apparatus as a unit into the borehole and to supply current to the ignition elements of the system, and wherein facilities are provided for insuring that the bottom gun section is always fired before the top gun section is fired.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

Fig. l is a side perspective view of a perforating gun assembly embodying the sequential firing system of the present invention;

Fig. 2 is a schematic diagram of a sequential firing system embodying the present invention;

Fig. 3 is a vertical sectional view on a larger scale taken substantially along the axis of two of the sub-assemblies which make up the gun assembly of Fig. 1, namely, the housing sub-assembly and the mounting sub-assembly which form the jet selector mechanism, certain of the parts being shown in elevation;

Fig. 4 is a fragmentary sectional view on a scale similar to Fig. 3 taken on a different axial plane through another sub-assembly, namely, the upper detonating cord sub-assembly;

Fig. 5 is a fragmentary detail view on a similar scale of a portion of the upper gun section;

Fig. 6 is a fragmentary view on a similar scale partly in section showing the details of one of the sub-assemblies disposed between the upper and lower gun barrels;

Fig. 7 is a view similar to Fig. 6 showingthe other of the sub-assemblies disposed between the upper and lower gun barrels; and

Fig. 8 is a similar view showing the cap member at the lower end of the gun.

Referring now more particularly to Fig. 1 of the drawing, the invention is there shown in its embodiment in an improved jet type perforating gun indicated generally at 10. In general, the perforating gun 10 comprises an upper gun section 11 and a lower gun section 12, the gun section 12 being provided with a lower end cap or closure member 13. The gun sections 11 and 12 are connected together by means of a bottom detonating cord sub-assembly 14, and a contact sub-assembly 15. At its upper end the gun section 11 is connected to an upper detonating cord sub-assembly 16, the upper end of the sub-assembly 16 being connected to a jet selector mechanism indicated generally at 17. The jet selector mechanism 17 comprises a housing sub-assembly 18 and a mounting sub-assembly 19, the sub-assembly 19 being connected to the upper end of the detonating cord sub-assembly 16. The upper end of the housing sub-assembly 18 is connected to the lower end of a cable head sealing subassembly 20, which in turn is connected to the lower end of a rope socket assembly 21. The latter assembly is employed to anchor the lower end of a cable 22, which is employed to lower the perforating gun 10 into and out of the cased well which is to be perforated. All of the described parts of the perforating gun are preferably secured together by means of threaded connections therebetween in order to provide a rigid over-all assembly and to facilitate assembly and disassembly of the gun structure.

Each of the gun sections 11 and 12 is provided with a plurality of explosion-ports ZS-Which are spaced apart along the length of each gun section. The ports 25 may also be spaced circumferentially of each gun section in any desired manner. A shaped jet-producing explosive charge is positioned within the gun barrel sections 11-. and

12 opposite each explosion port 25 so that the explosive forces developed upon detonation of the charge are discharged through the port. Preferably the explosive charges disposed in one of the gun barrel sections 11 or 12 are detonated substantially simultaneously by means of a detonating cord which extends longitudinally of the barrel and is threaded through the explosive charges. Also, it is preferred to employ an explosive charge supporting device of the type described and claimed in copending application Serial No. 209,972 filed February 8, 1951, Jacques H. Castel and assigned to the same assignee as the present invention, so that the explosive charges are accurately aligned with the explosion ports of the gun barrel regardless of relatively large variations in the physical dimensions thereof.

In accordance with the present invention an improved firing system is provided for sequentially firing the explosive charges of the gun sections 11 and 12. Specifically, the explosive charges disposed in the gun sections 11 and 12 are electrically ignited in sequence from the earths surface over an electrical circuit, schematically shown in Fig. 2, which comprises the single insulated conductor 23 of the cable 22. As shown, this circuit comprises a source of unidirectional potential indicated by the battery 30 and a source of alternating current 31 which are located at the earths surface. The unidirectional source 30 and the alternating current source 31 are adapted to be selectively connected to the cable 22 by means of a rotary selector switch indicated generally at 32. One side of each of the sources 30 and 31 is connected to ground along with the armored sheath of the cable 22 so that a closed return circuit is provided. The ungrounded side of the battery 30 is connected to a stationary contact 35 of the switch 32 which cooperates with the movable contact 36. The alternating current source 31 is connected through a potentiometer 40 to the other stationary contact 41 which also cooperates with the movable contact 36. The movable contact 36 is connected as shown to the single center conductor 23 of the cable 22. It will be understood that the usual facilities are provided for raising and lowering the cable 22 while maintaining electrica] contact with the central conductor 23 and sheath thereof.

In order to prevent the alternating current source 31 from being connected to the cable 22 before the unidirectional source 30 is connected thereto, the movable contact 36 may be arranged for movement only in a counterclockwise direction away from the illustrated normal or off position thereof. To this end, the movable contact 36 may be mounted for rotation with a rotor element 46 having a lug 46a engageable with a fixed stop 47 to prevent clockwise rotation of the movable contact 36 beyond the illustrated normal position thereof. The rotor element is preferably spring biased by a coil spring, not shown, to maintain the lug 46 in engagement with the fixed stop 46a, so that if the rotor element 46 and movable contact 36 are rotated to any off-normal position and then released they will be automatically returned to their normal positions. A knob 38 mounted on the rotor shaft may be used to rotate the rotor element 46 and movable contact 36 against the bias of the coil spring. With this arrangement, the-contacts 36 and 35 must be closed before the contacts 36 and 41 are closed, all in response to counterclockwise movement of the rotor element 46 and the movable contact 36. When the movable contact 36 is rotated in the counterclockwise direction from the normal or. off position illustrated, the contacts 35 and 36 are first closed, and upon further rotation these contacts are opened, and thereafter the contacts 41 and 36 are closed.

Withinthe perforating gunlt) thefiring circuit includes a. coupling transformer. 50 which is provided with. a

primary winding 51 having one end connected to the single conductor 23 of the cable 22 and the other end connected through the ignition element of a blasting cap 52 to ground potential through the walls of the perforating gun and the sheath of the cable 22. The blasting cap 52 is paralleled by a resistor 54 which is preferably positioned at a point removed from the blasting cap 52 so as to be physically isolated therefrom. One end of the secondary Winding 60 of the transformer 50 is connected to ground and the other end of this winding is connected through an isolating resistor 61 and the ignition element of a second blasting cap 62 to ground. A low resistance fuse 63 is connected from the ungrounded end of the winding 60 to ground in shunt with the resistor 61 and the ignition element of the blasting cap 62 in order to protect the cap 62 against accidental firing due to transient surges which may occur in the circuit when the battery 30 is connected to or disconnected from the circuit by the contacts 35 and 36. The blasting cap 52 is contiguous to the detonating cord associated with the explosive charges in the bottom gun section 12 and when the blasting cap 52 is ignited the detonating cord operates to fire all of the explosive charges in the gun section 12 substantially simultaneously. Likewise, the blasting cap 62 is contiguous to the detonating cord associated with the charges of the upper gun section 11 so that when the cap 62 is ignited the charges in the upper gun section of 11 are substantially simultaneously fired.

Considering now the operation of the above-described sequential firing system in sequentially firing the explosive charges of the gun sections 11 and 12 it will be understood that either of the gun sections 11 or 12 may be employed to perforate the well casing at a desired location. However, it is absolutely necessary to fire the charges of a particular gun section at the correct location in the borehole, since otherwise, the casing would be perforated at the wrong point, which always causes trouble and expense and may even result in the loss of the well. Accordingly, it is desirable to establish a sequence of firing and to correlate the positions of the gun sections with the firing sequence so that the point at which the casing is perforated is very accurately determined. In the illustrated embodiment of the present invention the bottom gun section 12 is always fired first, after which the perforating gun is moved to the desired perforating point and the top gun section 11 is fired. In order to prevent firing of the top gun when the bottom gun is fired, the above-described protective fuse is provided in shunt with the ignition element of the blasting cap 62 so that secondary currents which may be caused by switchlng transients, or the like, when the contacts of the switch 32 are opened and closed, are ineffective to fire the top gun section. Furthermore, the correct firing sequence is assured by providing the above-described means for preventing firing of the top gun section before the bottom gun section is fired. Thus with the selector switch 32 made to rotate only in a given direction from its normal or ofi setting in the manner described above the unidirectlonal source 30 is first applied to the cable 22 after which the alternating current source is connected thereto. In the alternative, the arm of the potentiometer 40 my be normally biased to its uppermost position by any suitable means so that clousre of the contacts 41 and 36 of the switch 32 will be ineffective to fire the top gun section without further adjustment of the potentiometer.

Considering now the manner in which the explosive charges of the bottom gun section 12 are fired, when the source 30 is connected to the cable conductor 23 through the contacts 35 and 36 of the selector switch 32, for example, direct current flows from the source 30 through the central conductor 23 of the cable 22, the primary winding 51 of the transformer 50 and the ignition element of the blasting cap 52 to the. grounded terminal of the source 30. The potential of the source 30 is so chosen that a direct current of sufficient magnitude flows through the ignition element of the blasting cap 52 to ignite the same so that the detonating cord associated with the cap 52 is ignited and substantially simultaneously fires the explosive charges of the bottom gun section 12. After the blasting cap 52 is ignited, the electrical circuit through this cap is broken, and without further provision for maintaining a closed circuit through the cable 22 it would be impossible to send further electrical currents through the cable and therefore to utilize the cable to fire the upper gun section 11. However, in accordance with one feature of the present invention this problem is obviated by providing the resistor 54 which is physically remote from the blasting cap 52 and is connected from the winding 51 to ground so that a closed circuit through the cable 22 is maintained. This resistor has a resistance many times greater than the resistance of the ignition element in the cap 52 so that it does not interfere with current flow through this ignition element to any appreciable extent.

After the bottom gun section 12 has been fired the perforating gun is moved to bring the upper gun section 11 into the desired perforating position, after which the latter gun section is fired. This is accomplished by first connecting the alternating current source 31 to the cable conductor 23 through the contacts 41 and 36 of the step selector switch 32, for example. When this is done, alternating current flows from the source 31 through the potentiometer 40, the conductor 23, the primary winding 51 of the transformer 50, and the resistor 54 to ground. The above-described flow of current through the primary winding 51 induces a voltage in the secondary winding 60 so that a secondary current flows through the circuit elements 61, 62 and 63 connected thereacross. As stated above, the resistance of the fuse 63 is very low as compared with the series resistance of the resistor 61 and the ignition element of the cap 62. Accordingly, the ignition element of the cap 62 is eifective- 1y by-passed for current flow therethrough until the fuse 63 is blown. Initially, current flow in the above-traced secondary current is not large enough to blow the fuse 63. However, by decreasing the resistance of the potentiometer 40, the secondary current is increased to a point at which the fuse 63 blows. By operating the potentiometer 40 to increase the current fiow in the secondary circuit of the transformer still further, the ignition element of the blasting cap 62 becomes sufiiciently energized to ignite the cap. When the blasting cap 62 is ignited,

the detonating cord associated therewith operates to fire the explosive charges of the upper gun section 11 substantially simultaneously. Thus with the described arrangement it is possible accurately to control the firing of each gun section and to be absolutely certain that the bottom gun section 12 is fired before it is possible to fire vthe upper gun section 11.

Considering now the structural arrangement of the present improved perforating gun in which the above-described sequential firing system is incorporated, it will be recalled that the gun selector mechanism 17 com- .prises an upper or housing sub-assembly 18 and a lower or mounting sub-assembly 19. Referring to Fig. 3, the housing sub-assembly 18 comprises a tubular body portion 70, the upper end of which is adapted to thread into the lower end of the cable head sealing sub-assembly and is provided with a top opening central aperture 71. The bottom end of the housing sub-assembly body portion 70 is adapted to thread onto the upper end of the mounting sub-assembly 19 and is provided with a downwardly opening central bore 72 of relatively large diameter and a smaller central bore 73.

The mounting sub-assembly 19 is provided with a relatively short, generally tubular body portion 82 having an .upper threaded end which is adapted to be threaded into the body portion 70 of the housing sub-assembly 17 and ;a lower threaded portion which is adapted to be joined to the upper detonating cord sub-assembly 16 by means of 6 the threaded sleeve 83 (Fig. 4). An Curing 84 of resilient packing material is positioned between the sub-assemblies 18 and 19 to provide a water-fight seal between these units. 1

Electric contact with the central conductor 23 of the cable 22 is made by means of a conductive receptacle 74, which extends through the partition 75 between the cavities 71 and 73 and is insulated therefrom by means of the insulating sleeve '76. The bottom end of the receptacle 74 is threaded into another conductive receptacle 77, which is adapted to receive a conductive plug member 78 which projects upwardly from the top of the mounting sub-assembly 19.

The mounting sub-assembly 19 carries the transformer 50, and in order to insulate the transformer from the body portions 70 and 82, respectively, of the sub-assemblies 18 and 19, there is provided a bottom housing member 79 of insulating material and a top housing member 80 of insulating material which completely encloses the transformer 50 and are held together by means of the screws 81. The housing members 79 and 80 are positioned atop the mounting sub-assembly 19 by means of the bolts 85 which thread into the top of the body portion 82 of the mounting sub-assembly 19. The conductive plug 78 is positioned in the upper wall of the housing member 80 and a lead wire 86 connects the plug '78 to one side of the primary winding of the transformer 50. The other end of the primary winding of the transformer 50 is connected through the resistor 54, which is positioned immediately above the upper housing member 80, to the bolt 85 which is in turn connected to the body portion 82 of the sub-assembly 19. In this connection it will be understood that the body portions of all of the sub-assemblies 11, 12, 14, 15, 16, 18 and 19 are threaded together and are electrically connected to the outer sheath of the cable 22 so as to be effectively at ground potential.

In order to connect the transformer 50 with the blasting caps 52 and 62 positioned respectively in the units 16 and 14 of the perforating gun shown in Fig. l, the conductive plug members 88 and 89 are provided which are positioned in the bottom wall of the lower housing member 79. The plug member 88 is connected through the lead wire 90 to the ungrounded side of the secondary winding 60 of the transformer 50 and the plug member 89 is connected to the primary winding 51 of the transformer 50. A pair of locating pins, one of which is shown at 91, are threaded into the bottom wall of the housing member 79 for the purpose of centering the plug members 88 and 89 when the enclosed transformer is assembled on top of the sub-assembly 19. The plug member 88 is insertable into a conductive receptacle provided with a shank 96 which extends through the sub-assembly 19 and connects with a conductive plug 97 projecting below the end of the sub-assembly 19. The receptacle 95, shank 96, and plug 97 are insulated from the body portion 82 of the sub-assembly 19 by means of the insulating sleeves 98, 99 and 100. In a like manner the plug member 89 is conductively connected to the plug member 101 and is insulated from the body portion 82 of the sub-assembly 19. Locating pins 102 project from the bottom of the sub-assembly 19 so as to position the plug members 97 and 101 over cooperating receptacles in the sub-assembly 16 when these units are assembled.

The upper detonating cord sub-assembly 16 as shown in Fig. 4 comprises a generally tubular body portion 105 which is threaded at the upper end to receive a coupling sleeve 83 and is provided with a central bore 106 therein. The bore 106 is adapted to receive the body portion of a plug member 107, also of conductive material with the flange 118 of the plug 107 resting on the top rim of the body portion 105. The member 107 carries a pair of conductive receptacles 108 and -109,"adapted respectively to receive the plug members spring 147 when the sub-assemblies 14 and 15 97 and 101 of the mounting sub-assembly 19 and supports the resistor 61, blasting cap 62 and fuse 63 within the bore 106. A pair of sealing rings 110 and 111 of resilient packing material, are provided for sealing the flange 118 to the sub-assemblies 19 and 16 in a watertight manner. The receptacles 108 and 109 are insulated from the plug 107 by means of the insulating sleeves 112 and 113 and are provided with shanks 114 and 115 which extend through the plug member 107 and thread into conductive terminal studs 116 and 117. The blasting cap 62 is in contact with the upper end of a detonating cord 120, which may be of Primacord or the like, and extends through a narrow central bore 121 provided in the body portion 105. The circuit elements 61 and 63 are connected to the terminal stud 116 by any suitable means, such as by soldering or the like, the elements 61, 62 and 63 are connected together in the manner shown in the schematic diagram of Fig. 2, and there is provided a ground terminal 122 which is threaded into the plug member 107 and grounds one end of the fuse 63 and the blasting cap 62.

In order to connect the conductive terminal stud 117 to the blasting cap 52 of the bottom gun section 12, there is provided the conductor 125, which is connected to the stud 117 by means of the screw 126 and extends through the bore 121 along with the detonating cord 120. The bottom end of the upper detonating cord subassembly 16 threads into the upper end of the top gun section 11 as shown in Fig. 4. In this connection it will be understood that the detonating cord 120, which extends longitudinally of the gun section 11, is threaded through the explosive charges disposed in this gun section so that when the upper end of the detonating cord 120 is ignited all of the explosive charges of the gun section 11 are fired at substantially the same time. Thus, considering Fig. of the drawings, the detonating cord 120 is adapted to thread through one end of each explosive charge 130. The gun section 11 comprises a tubular gun barrel 135, which is provided with the spaced explosion ports 25 in the walls thereof, the upper end of which is adapted to thread into the upper detonating cord sub-assembly 16 and the lower end of which is adapted to thread into the contact sub-assembly 15. The conductor 125, which connects the primary winding of the transformer 50 with the blasting cap 52, extends through the length of the gun section 11 and past the explosive charges 130 without making electrical contact therewith.

As shown in Fig. 6 of the drawings, the conductor 125 is connected at its bottom end to a conductive terminal 140, which is positioned in an insulating sleeve 141 seated in the upper end of the body portion 142 of the contact sub-assembly 15. The upper end of the body portion 142 is adapted to be threaded into the lower end of the gun section 11 and the bottom end of the body portion 142 may be threaded onto the upper end of the bottom detonating cord sub-assembly 14. One end of a conductive rod 145 is threaded into the bottom of the terminal 140 and is provided at the other end thereof with a recessed head portion 146. The head portion 146 is adapted to receive the upper end of a coil are connected together and the lower end of the spring 147 rests upon an insulating ring 148 carried in a conductive plug member 150 positioned atop the sub-assembly 114 and extending into the bore 144 thereof. A conductor 151 is connected to. the spring 147 by means of a terminal screw 148 and the other end of the conductor is connected to one side of the blasting cap 52, which is positioned within the bore 144, the other side of the blasting cap 52 being connected through the conductor 152v to. a ground terminal screw 153'which is threaded into the plug member 150.

The blasting cap 52 is in contact with the upper end of a detonating cord 155 which extends downwardly through the bore 144 of the sub-assembly 14 and through the length of the lower gun section 12 as shown in Figs. 7 and 8 of the drawings. The detonating cord 155 is threaded through the explosive charges of the lower gun section 12 in a manner identical to that described above in connection with the upper gun section 11. The upper end of the gun section 12 is adapted to thread onto the bottom end of the sub-assembly 14 and the lower end of the gun section 12 threads onto a bottom cap member, or bull plug 13. A sealing ring 156 of resilient packing material is positioned between the bottom end of the gun section 12 and the cap 13 to effect a water-tight seal.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention as defined in the appended claims.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. In combination with a jet type perforating gun which comprises a plurality of shaped charges and a cable for lowering said gun into a borehole and including a single insulated conductor, a system for sequentially firing said charges, comprising means for non-concurrently transmitting direct and alternating currents over said conductor to said gun, a coupling transformer in said gun and having one winding thereof connected in series with said conductor, means included in said gun and terminating said cable for firing at least one explosive charge of said gun in response to said direct current, means for maintainingia closed circuit including said one winding and said conductor after said one charge is fired, and means in said gunand connected to another winding of said transformer for firing at least one other explosive charge of said gun in response to said alternating current.

2. In combination with a jet'type perforating gun which comprises a plurality of shaped charges and a cable for lowering said gun into a borehole and including a single insulated conductor, a system for sequentially firing said charges, comprising means for non-concurrently transmitting direct and alternating currents over said conductor to said gun, a coupling transformer in said gun and having one winding thereof connected in series with said conductor, means included in said gun and terminating said cable for firing at least one explosive charge of said gun in response to said direct current, means for maintaining a closed circuit including said one winding and said conductor after said one charge is fired, means in said gun and connected to another winding of said transformer for firing at least one other explosive charge of said gun in response to said alternating current, and means for preventing firing of said other charge in response to transient surges produced in said other winding upon connection and disconnection of said direct current to said conductor.

3. In combination with a jet type perforating gun which comprises a plurality of shaped charges and a cable for lowering said gun into a borehole and including a single insulated conductor, a system for sequentially firing said charges, comprising means at the earths surface for nonconcurrently transmitting direct and alternating currents over said conductor to said gun, a transformer in said gun and having one winding thereof connected in a series with said conductor, means in said gun and connected in series with said one winding for firing at least one explosive charge of said gun in response to said direct current, means in said gun and connected across another winding of said transformer for firing at least one other charge of said gun in response to alternating current fiow of predetermined magnitude therethrough, means at the earths surface for varying the magnitude of said alternating current, fuse means connected across said last named firing means and adapted to delay the firingthereof until said alternating current reaches said predetermined magnitude, and a resistor connected across said first named firing means and unaffected by the firing thereof, thereby to permit the flow of said alternating current over said conductor after said one charge is fired.

References Cited in the file of this patent UNITED STATES PATENTS 2,264,450 Mounce Dec. 2, 1941 10 Foster Jan. 5, 1943 Robidoux Jan. 11, 1944 D011 Nov. 21, 1944 Schlumberger Oct. 21, 1947 Muskat Jan. 10, 1950 FOREIGN PATENTS Germany Apr. 28, 1915 

